Solar Wind Rips Up Martian Atmosphere

Nov.
21, 2008: Researchers have found new evidence that
the atmosphere of Mars is being stripped away by solar wind.
It's not a gently continuous erosion, but rather a ripping
process in which chunks of Martian air detach themselves from
the planet and tumble into deep space. This surprising mechanism
could help solve a longstanding mystery about the Red Planet.

"It
helps explain why Mars has so little air," says David
Brain of UC Berkeley, who presented the findings at the 2008
Huntsville Plasma Workshop on October 27th.

Billions
of years ago, Mars had a lot more air than it does today.
(Note: Martian "air" is primarily carbon dioxide,
not the nitrogen-oxygen mix we breathe on Earth.) Ancient
martian lake-beds and river channels tell the tale of a planet
covered by abundant water and wrapped in an atmosphere thick
enough to prevent that water from evaporating into space.
Some researchers believe the atmosphere of Mars was once as
thick as Earth's. Today, however, all those lakes and rivers
are dry and the atmospheric pressure on Mars is only 1% that
of Earth at sea-level. A cup of water placed almost anywhere
on the Martian surface would quickly and violently boil away—a
result of the super-low air pressure.

Above,
right: An artist's concept of ancient Mars with abundant
air and water. [Larger
image]

So
where did the air go? Researchers entertain several possibilities:
An asteroid hitting Mars long ago might have blown away a
portion of the planet's atmosphere in a single violent upheaval.
Or the loss might have been slow and gradual, the result of
billions of years of relentless "sand-blasting"
by solar wind particles. Or both mechanisms could be at work.

Brain
has uncovered a new possibility--a daily ripping process intermediate
between the great cataclysm and slow erosion models. The evidence
comes from NASA's now-retired Mars Global Surveyor (MGS) spacecraft.

In
1998, MGS discovered that Mars has a very strange magnetic
field. Instead of a global bubble, like Earth's, the Martian
field is in the form of magnetic umbrellas that sprout out
of the ground and reach beyond the top of Mars' atmosphere.
These umbrellas number in the dozens and they cover about
40% of the planet’s surface, mainly in the southern hemisphere.

For
years, researchers thought the umbrellas protected the Martian
atmosphere, shielding pockets of air beneath them from erosion
by the solar wind. Surprisingly, Brain finds that the opposite
can be true as well: "The umbrellas are where coherent
chunks of air are torn away."

Addressing
his colleagues at the Workshop, he described how he made the
discovery just a few months ago:

Brain
was scrolling through archival data from Global Surveyor's
particles and fields sensors. "We have measurements from
25,000 orbits," he says. During
one of those orbits, MGS passed through the top of a magnetic
umbrella. Brain noticed that the umbrella's magnetic field
had linked up with the magnetic field in the solar wind. Physicists
call this "magnetic reconnection." What happened
next is not 100% certain, but Global Surveyor's readings are
consistent with the following scenario: "The joined fields
wrapped themselves around a packet of gas at the top of the
Martian atmosphere, forming a magnetic capsule a thousand
kilometers wide with ionized air trapped inside," says
Brain. "Solar wind pressure caused the capsule to 'pinch
off' and it blew away, taking its cargo of air with it."
Brain has since found a dozen more examples. The magnetic
capsules or "plasmoids" tend to blow over the south
pole of Mars, mainly because most of the umbrellas are located
in Mars' southern hemisphere.

Above:
Dave Brain of UC Berkeley presented this slide at the 2008
Huntsville Plasma Workshop to explain in cartoon fashion how
plasmoids carry air away from Mars. [Larger
image]

Brain
isn't ready to declare the mystery solved. "We're still
not sure how often the plasmoids form or how much gas each
one contains." The problem is, Mars Global Surveyor wasn't
designed to study the phenomenon. The spacecraft was only
equipped to sense electrons, not the heavier ions which would
make up the bulk of any trapped gas. "Ions and electrons
don't always behave the same way," he cautions. Also,
MGS sampled the umbrellas at fixed altitudes and at the same
local time each day. "We need to sample many altitudes
and times of day to truly understand these dynamic events."

In
short, he told the audience, "we need more data."

Brain
is pinning his hopes on a new NASA mission named MAVEN. Short
for "Mars Atmosphere and Volatile Evolution," MAVEN
is an upper atmosphere orbiter currently approved for launch
to Mars in 2013. The probe is specifically designed to study
atmospheric erosion. MAVEN will be able to detect electrons,
ions and neutral atoms; it will be able to measure both magnetic
and electric fields; it will travel around Mars in an elliptical
orbit, piercing magnetic umbrellas at different altitudes,
angles, and times of day; and it will explore regions both
near and far from the umbrellas, giving researchers the complete
picture they need.

If
magnetized chunks of air are truly being torn free, MAVEN
will see it happening and measure the atmospheric loss rate.
"Personally, I think this mechanism is important,"
says Brain, "but MAVEN may yet prove me wrong."

Meanwhile,
the Mystery of the Missing Martian Air is shaping
up to be a ripping good yarn.